微-纳米受限空间原油-天然气最小混相压力预测方法

Prediction method for the minimum miscibility pressure of crude oil and natural gas in micro-nano confined space

微-纳米受限空间原油-天然气的最小混相压力(MMP)对致密/页岩油储层注天然气提高采收率参数优化、方案设计及产能预测至关重要,但至今尚缺少获取该参数的可靠方法。针对这一关键问题,提出一种微-纳米受限空间原油-天然气界面张力(IFT)计算方法,通过原油-天然气相平衡计算得到Parachor模型参数从而计算出IFT,并基于界面张力消失法(VIT)预测MMP。该方法改进了气液平衡计算及Peng-Robinson状态方程,考虑了毛细管压力并修正了流体的临界压力和临界温度,简化了传统计算方法,并将气/液相压力考虑到平衡常数Ki的迭代公式中。通过设计8组不同组分天然气并以长庆油田原油和拟注入天然气为实例,分别计算了孔隙半径r_(p)为5000 nm、1000 nm、500 nm、100 nm、10 nm时的IFT和MMP。计算结果表明,压力越高,IFT越小;在r_(p)为100~5000 nm时,原油-天然气体系的IFT和MMP基本不受r_(p)的影响;当r_(p)≤100 nm时,IFT和MMP显著降低,MMP最大降幅达38.76%,说明致密/页岩油储层注天然气提高采收率相比常规储层注天然气更容易混相。原油-天然气体系的MMP与C2—C4含量呈现负相关性,且r_(p)越小,MMP递减速度越快,最大降幅达57.33%(r_(p)=10 nm,C2—C4摩尔分数增至40%)。将计算结果与文献报道数据进行对比,进一步验证了模型的可靠性。

The minimum miscibility pressure(MMP)of crude oil-gas in micro-nano confined space is crucial to the optimization of natural gas EOR parameters,scheme design and production prediction in tight/shale reservoirs.However,there is still no reliable method for obtaining this parameter up till now.To address this key issue,the paper proposes a micro-nano confined space crude oil-gas interfacial tension(IFT)calculation method;the Parachor model parameters are obtained by crude oil-gas phase equilibrium calculations,so as to further obtain IFT and predict MMP based on the vanishing interfacial tension(VIT)method.This method improves the vapor-liquid equilibrium(VLE)calculation and the Peng-Robinson equation of state(PR-EOS),considers the capillary pressure and corrects the critical pressure and critical temperature of the fluid,simplifies the conventional calculation method,and takes into account the vapor and liquid phase pressures in the Ki iterative formulation.Using Changqing crude oil and quasi-injected natural gas as the samples,the IFT and MMP were calculated when the pore radius r_(p) equals to 5000 nm,1000 nm,500 nm,100 nm,and 10 nm for eight groups of different components of natural gas.The results show that the higher the pressure,the smaller the IFT;when r_(p)ranges from 100 nm to 5000 nm,the IFT and MMP are basically unaffected by r_(p);when r_(p)≤100 nm,IFT and MMP decrease significantly,and the maximum decrease of MMP is 38.76%,indicating that the miscibility of crude oil and natural gas in tight/shale oil reservoirs,as compared with that in conventional reservoirs,can be more readily achieved.There is a negative correlation between the MMP of crude oil-gas system and C2-C4 content,and the smaller the r_(p),the faster the decreasing rate of MMP,with the maximum decrease of 57.33%(r_(p)=10 nm,C2-C4 content increases to 40 mol%).Finally,the calculated results were compared with the data reported in the literaturem to verify the reliability of the model.